The present invention relates an electron cyclotron resonance ion source with coaxial injection of electromagnetic waves, more particularly making it possible to produce multicharged ions.
It has numerous applications as a function of the different values of the kinetic energy of the ions produced, in the field of ion implantation, micro etching and more particularly in particle accelerator equipment used in both the scientific and medical fields.
In electron cyclotron resonance ion sources, the ions are obtained by ionization in a sealed enclosure, such as an ultra-high frequency cavity, of a gaseous medium constituted by one or more metal vapours or gases using electrons greatly accelerated by electron cyclotron resonance.
Electron cyclotron resonance is obtained through the combined action of a high frequency electromagnetic field injected into the enclosure and an axial symmetry magnetic field created in the enclosure. This axial magnetic field, which has an amplitude increasing from the centre of the enclosure towards its ends, has in particular an amplitude B.sub.r satisfying the electron cyclotron resonance condition B.sub.r f.2.pi.m/e in which e represents the charge of an electron, m its mass and f the frequency of the electromagnetic field. This axial magnetic field is generally created by solenoids or magnetic coils surrounding the enclosure.
In this type of ion source, the quantity of ions which can be produced results from the competition between two processes, on the one hand the formation of the ions by electron impact on neutral atoms constituting the gaseous medium to be ionized and on the other hand by the destruction of these same ions by recombination during a collision of said ions with a neutral atom. This neutral atom can come from atoms of the gaseous medium which have not yet been ionized, or can be produced by the impact of an ion on the walls of the enclosure.
To minimize the destruction of the ions formed, the latter are confined in the enclosure, together with the electrons used for the ionization of the neutral atoms, thereby reducing collisions of the ions and electrons with the enclosure wall. For this purpose, within said enclosure is formed a radial magnetic field, which is superimposed on the axial magnetic field. The superimposing of these magnetic fields defines in the enclosure at least one closed "equimagnetic" surface having no contact with the enclosure walls. This surface represents the location of the points where the amplitude of the magnetic fields has the same value.
The radial magnetic field is in particular produced by magnetized bars arranged symmetrically around the enclosure and each constituted by several joined elementary magnets.
FIGS. 1a and 1b diagrammatically represent an example of a known electron cyclotron resonance ion source, which is described in FR-A-2 553 574, filed on Oct. 17, 1983 by the same Applicant. This ion source comprises an enclosure 2 within which a high vacuum has been formed, said enclosure constituting a resonant cavity which can be excited by a high frequency electromagnetic field. The latter is produced by an electromagnetic wave generator 3, such as a Klystron supplied with current by a power supply 6. This field is introduced into enclosure 2 by a wave guide 4, such as a metal duct.
This ion source also comprises means 10, indicated in mixed line form, making it possible to produce an axial magnetic field and a radial magnetic field within enclosure 2. These magnetic fields make it possible to define an equimagnetic closed surface 11.
In order to ionize a gas, the latter is introduced into enclosure 2 by a duct 8. The association of the axial magnetic field and the electromagnetic field makes it possible to highly ionized the gas introduced into the enclosure. The electrons produced are then highly accelerated by electron cyclotron resonance, which leads to the formation of a plasma of hot electrons confined in surface 11.
In the case where ions are produced from a solid and in particular a metallic sample 12, the latter is fixed to a support 14 in enclosure 2, in the vicinity of surface 7. Solid sample 12 is firstly vaporized and the vapours obtained are ionized, as in the case of a gas. As described hereinbefore, a hot electron plasma forms in the surface 11.
The vaporization of the solid sample is due to the interaction of the hot plasma with the sample. On starting of the vaporization reaction, the hot plasma necessary can be produced by ionizing a gas introduced into enclosure 2 by duct 8. This gas is solely injected to start of the vaporization reaction and the hot plasma necessary for maintaining the vaporization reaction then being obtained from the actual solid sample.
No matter what type of sample is used, the ions formed in the enclosure are extracted therefrom, e.g. by an extraction electric field generated by a potential difference created between a revolution electrode 16 and enclosure 2, electrode 16 and the enclosure 2 being connected to a power supply 17.
In order to obtain a constant intensity ion current, the latter is regulated by a regulating and control device.
FIGS. 1a and 1b respectively show an embodiment of the control and regulating device, which comprises means 18 using an electric and/or magnetic field for analysing the ions from enclosure 2. This device also comprises a motor 20, connected via a rod 22 to support 14 of solid sample 12, making it possible to slowly displace the latter in such a way that it intercepts in an optimum manner the plasma confined in surface 11. The more the solid sample 12 penetrates the enclosure 2, the higher its temperature and vaporization level.
This device also comprises a pulse generator 24 connected to the power supply 6. By adjusting the cycle, i.e. the ratio between the duration of a pulse and the pulse period, this pulse generator makes it possible to control the power supply 6 supplying the electromagnetic wave generator 3. Thus, the control of the mean power of the electromagnetic field is obtained by pulsating it.
Moreover, for regulating the ion current leaving enclosure 2, the total pressure in the enclosure must be kept constant. Total pressure measuring means 28 connected to enclosure 2, such as a pressure gauge make it possible via an appropriate means to ensure the operation of a valve 26 connected to the gas introduction duct, so that the total pressure in the enclosure remains constant. The appropriate means can, as shown in FIG. 1a, be a comparator 30 or, as shown in FIG. 1b, a microprocessor 32.
Comparator 30 is connected to means 28 and to valve 26, a reference voltage R being applied to said comparator.
Microprocessor 32 is connected to means 34 for measuring the intensity of the extracted ion current, to means 28, to valve 26, to motor 20 and to pulse generator 24. Thus, said microprocessor 32 permits an automatic regulation of the ion current.
FIGS. 2a and 2b diagrammatically show a known device making it possible to produce multicharged ions through a shielded magnetic structure. This shield makes it possible to only magnetize the volume useful for electron cyclotron resonance in enclosure 1. The device shown in FIGS. 2a and 2b is described in EP-A-O 138 642 filed on Aug. 17, 1984 in the name of the same Applicant.
This device comprises permanent magnets 35 fixed to the inner wall of a cylinder 37 of a ferromagnetic material, solenoids 39 arranged on either side of cylinder 37 and a magnetic shield 41. A material 43 makes it possible to magnetically isolate cylinder 37 from shield 41.
The permanent magnets 35 distributed in accordance with the circular section of cylinder 37 (FIG. 2a) can be quadripolar, hexapolar, octopolar, etc (FIG. 2b). These permanent magnets realise a multipolar radial magnetic field 45, whilst coils 39 supply and axial magnetic field 49. the superimposing of these two magnetic fields produces a closed equimagnetic surface 11.
Such a known device makes it possible to obtain a magnetically shielded, opaque ion source, whose magnetic axis 50 coincides with that of solenoids 39 and cylinder 37. This magnetic axis 50, which is also the longitudinal axis of the device, traverses shield 41 through two openings 51, 53 made therein, so as to permit on the one hand the extraction of ions from enclosure 1 and on the other hand the introduction of electromagnetic waves and the introduction of the sample into enclosure 1.
The axial injection of the electromagnetic waves into the enclosure causes certain problems. Thus, there is no magnetic field upstream of enclosure 1 level with axial opening 53. This absence of a magnetic field does not make it possible to easily guide the electromagnetic waves towards enclosure 1, as in the case of the attached FIGS. 1a and 1b, where the electromagnetic waves enter the enclosure in a relatively uniform magnetic field.
Moreover, at axial opening 53 located in the magnetic shield, the electromagnetic waves must pass through a resonance zone, where the modulus of the magnetic field passes suddenly from a zero value to a maximum value.
Moreover, the longitudinal axis 50 of enclosure 1 is not available as a result of the axial introduction of the electromagnetic waves. Thus, it is not possible to directly associate with said ion source a device for controlling and regulating the extracted ion current, as described relative to FIGS. 1a and 1b.
The object of the present invention is consequently to obviate these disadvantages by providing an ion source with coaxial injection, comprising a transition cavity and a group of ducts making it possible to guide the electromagnetic waves towards to enclosure and to inject them into the latter along its longitudinal axis, whilst still leaving said axis available.